Getting a MS while in med school is like pissing your pants while wearing dark trousers: it will give you a good, warm feeling, but almost no one else will notice. That being said, I think you should go ahead and do it since you seem to love research. Even if it does not lead to anything more career-wise, it will allow you to do something your really like with a minimum of inconvenience. (Your next chance to pursue a passion fulltime for a year will be about 45 years from now, when you retire.)